The present invention relates to communication systems and protocols for voice communications and, more particularly, to a communication system and protocol for transmitting and receiving voice calls over the Internet where at least one terminal is a mobile terminal.
Mobile wireless communications have evolved into a well-established field with numerous industry standards for both analog and digital communication systems. In digital wireless communication systems, undue degradation to the voice quality can be substantially prevented even though the digital data suffers from uncorrected transmission errors. For example, in British Patent No. 8,119,215 to Applicant, a method of detecting errors in frequency-hopping transmissions of digital voice data is described, wherein the data from hops determined to be in error is replaced by data deemed not to be in error. The new data preferably corresponds to the voice signal one or more whole pitch periods earlier or later in time. Since the introduction of this technology, digital mobile communication systems have employed bad-data replacement to maintain the highest possible voice quality under marginal radio conditions. Such methods are variously known as xe2x80x9cdeburping,xe2x80x9d or more scientifically, as xe2x80x9cartificial parameter reconstructionxe2x80x9d (APR). The detection of uncorrected errors and their handling by APR typically occurs immediately upon reception. Moreover, the compressed digital voice formats used for wireless transmission are generally converted to standard Pulse Coded Modulation (PCM) or analog voice waveforms for further transmission over the telephone network.
In prior art digital mobile communication systems, it is also known to use digital encryption over the wireless communication link, which is otherwise too easily intercepted. A user specific encryption key is typically established for each call at the mobile terminal and at the base station serving as the present host for the mobile terminal. The encrypted data transmitted from the mobile terminal is then deciphered at the base station so that it is possible to manipulate the plain text voice bits in the APR algorithm. After deciphering, the base station may pass the call to the Public Switched Telephone Network (PSTN) so that the intended recipient may receive the call. The transmission over the PSTN or other wire-based network is generally unencrypted. Consequently, communications remain vulnerable to interception on the PSTN or other wire-based network. In theory, such interception is only legally possible by law enforcement agencies, but there are no safeguards against illegal interception. Thus, sensitive information may actually be better protected during a wireless transmission than a wire-based transmission.
With increasing amounts of sensitive financial, personal, technical, and business information being transmitted over the Internet, there will be increased concern for security and greater use of encryption. Imminent changes to government policy are likely to sacrifice access by law enforcement agencies to wire-based networks for the greater benefits of security in general, thus encouraging more encryption on wire-based networks. An initiative promoting this attitude is known as Safety and Freedom through Encryption, or SAFE for short. However, to date, there has been no real effort to provide mass encryption of communications over wire-based networks.
Another recent development is the use of the Internet for voice calls, known in the industry as xe2x80x9cIP voice.xe2x80x9d Since Internet access is essentially a subscription-charge-only service, with no usage charge for minutes or long distance, this arrangement is attractive to subscribers. To date, IP voice has only been used for calls between fixed subscribers, i.e., those connected to wire-based networks.
Mobile subscribers can be connected to fixed subscribers using IP voice. However, the protocol over the wireless communication link is normally the standard circuit-switched wireless protocol. Conversion to Internet Protocol (IP) is done at a wireless network station, such as a base station. In the future, mobile communication systems known as third generation mobile communication systems will allow the Internet Protocol (IP) to be extended over the wireless link. For example, GSM""s General Packet Radio Service (GPRS) and a development thereof known as EDGE, are currently being planned and implemented. IP is an end-to-end protocol. The transport mechanism is, in principle, a bit-exact transport mechanism, meaning that the protocol ensures that all bits are transmitted correctly. If encryption is used, it is end-to-end encryption. It will be appreciated that, if traffic is end-to-end encrypted, it is impossible for the wire-based network to determine whether a given quantity of bits represents speech or computer data, and therefore, it is impossible to have differential pricing. Likewise, it may be difficult to perform APR in the mobile communication network, which requires modification of plain text data.
To date, IP voice is in an embryonic stage of development that has been conceived for calls between fixed terminals, without taking into account the specific needs of wireless communications and the implication of bit-exact protocols or end-to-end encryption. Thus, there is a need for an IP voice protocol that accommodates wireless communication and end-to-end encryption and that is compatible with existing mobile communication technologies.
The present invention overcomes the disadvantages of prior art IP voice networks by accommodating the needs for wireless communications while still allowing end-to-end encryption. According to the present invention, when a digital speech signal is formatted for transmission over a wireless communication link, a blank field, referred to herein as an error indication field, is appended to each speech data packet. The bits of the error indication field are initially set to a value indicative of no error. The speech bits and error indication bits in the speech data packet are then separately encrypted, for example, by bit-wise encryption. Error correction and error detection coding are performed following encryption. By performing error coding after encryption, it is possible to detect errors at the receiving end without first decrypting the message. Also, the use of bit-wise encryption allows bits in the error indication field to be inverted to give an error indication without decrypting the message. Thus, the present invention allows the error indication to be added to an already-encrypted message without altering the total number of bits contained in the message. The ability to add an error indication to an already-encrypted message without changing the total number of bits allows use of end-to-end encryption and other end-to-end protocols, such as the Internet Protocol (IP), without causing protocol stack violations.
In one embodiment, a system for transmitting speech from a fixed terminal to a mobile terminal is disclosed. Analog speech is received at a fixed terminal, converted to a digital signal, and then turned into speech bits by a vocoder. The speech bits are transmitted in a speech frame having an error indication field. The bits of the error indication field are initially set to a default value indicative of no error. The speech bits and error indication bits are encrypted, preferably with bitwise encryption technology, to generate an encrypted speech signal. The encrypted speech signal is then formatted for transmission over the Internet to a wireless base station. The wireless base station strips the Internet formatting from the message and performs error detection coding and error correction coding on the encrypted speech signal. Then, the base station formats a message for transmission over the wireless communication link to the mobile terminal. The mobile terminal receives and demodulates the message, and then performs error correction and detection decoding on the still-encrypted signal. An error indication is generated. The resulting speech bits and error indication field are then decrypted and passed to a vocoder prior to digital-to-analog conversion for emission through a speaker on the mobile terminal.
In another embodiment, a system for transmitting voice from a mobile terminal to a fixed terminal or another mobile terminal is disclosed. A vocoder in the mobile terminal digitizes and codes speech. An error indication field is appended to the speech bits output by the vocoder. An encoder in the mobile terminal performs error detection and error correction encoding on the speech bits and error indication bits to protect against wireless transmission errors. A transmitter formats the encoded bits for transmission to a base station. A base station receiver receives the transmitted packets, decodes the error correction and error detection codes, and determines if a transmission error occurred. If an error is detected, the bits in the error indication field are altered to give an error indication. The base station then reformats the decoded data, including the bits in the error indication field, for onward transmission over the Internet.
The invention further comprises a terminal for receiving digital data packets including an error indication. The terminal processes the received digital data packets to reformat them for input to a vocoder. If the error indication field contains an indication of no error, the speech bits are processed normally by the vocoder. If the error indication field contains an indication of error, alternative processing, such as Artificial Parameter Reconstruction, is used to mask or conceal the erroneous data.
In a further aspect of the invention, a wireless base station for transmitting voice over a wireless communications link to a mobile terminal is provided. The base station receives the digital voice messages as packet data over the Internet. The messages may originate at either a fixed terminal or a mobile terminal. The data packets include an error indication field. The wireless base station reformats the digital voice messages for wireless transmission to the mobile terminal and performs error correction and error detection coding. The intended mobile terminal receives the reformatted data, error decodes the message and uses the error detection code to determine the presence or absence of wireless transmission errors. If the error detection code indicates the presence of uncorrected transmission errors, the mobile terminal reformats the received data to replace erroneous data with other data. Alternatively, if the error detection code indicates no uncorrected errors, but the decoded blank field is indicative of errors occurring nearer the source, the mobile terminal also replaces the erroneous data. If the data is not erroneous, non-erroneous data and replaced erroneous data are passed to a voice decoder for reconstructing the voice signal.
In yet another implementation of the invention, a base station is provided for transmitting digital voice data received over the Internet, or other fixed voice network, to a mobile terminal or a fixed terminal. An error indication field that is used to provide an error indication is appended to the voice data. The base station performs error detection and error correction coding and reformats the message for transmission to the mobile terminal. Data determined to be in error by the base station may optionally be replaced by other data before onward transmission to the mobile terminal. This may occur, for example, when the message is received at the base station from a mobile terminal. At the receiving mobile terminal, error correction decoding and error detection are performed. If uncorrected wireless transmission errors are detected, the erroneous data is replaced by other data.
Another aspect of the invention relates to the provision of end-to-end encryption of voice messages over the Internet when at least one subscriber is using a mobile terminal. Voice for transmission from a mobile terminal is converted to a digital bitstream by a vocoder, and an error indication field containing at least one error indication bit is appended. The digital bitstream is then encrypted independently of the error indication field, which may or may not be separately encrypted, by addition of an encryption sequence calculated from a secret key. The encrypted speech bits and the error indication bits are then protected by error correction and error detection encoding, including possible unequal error protection in dependence on an importance-ranking of the bits, and formatted into packets for transmission to a wireless base station. At the wireless base station, the received packets are error correction decoded and the error detection code is processed to indicate the presence or absence of uncorrected wireless transmission errors. If uncorrected wireless transmission errors are indicated, the error indication field is modified by inverting at least one error indication bit. The decoded data packets, including the modified error indication field, are then transmitted over the Internet to a fixed subscriber or to another wireless base station and thence to another wireless subscriber. The receiving terminal receives, decodes, and decrypts the transmitted packets including the error indication field using the same secret key. If the error indication field indicates wireless transmission errors, the decrypted digital voice data containing the errors is replaced by other data computed from current, past, or future non-erroneous data.